Air motor controller with selfbalancing potentiometer



Jan 20, 1953 sc n- 2,625,911

7 AIR MOTOR CONTROLLER WITH SELF'BALANCING POTENTIOMETER Filed March 29, 1949 2 SHEETS-SHEET l FIG.I

INVENTOR. HENRY M. SCHMITT WK/QZQM ATTORNEY.

Jan. 20, 1953 sc rrr 2,625,911

AIR MOTOR CONTROLLER WITH SELF"BAIJRJCIIJC-l PQTENTIOMETER Filed March 29, 1949 2 SHEETS-SHEET 2 FIG. 4

INVENTOR. HENRY M. SCHMITT ATTORNEY.

Patented Jan. 20, 1953 AIR MOTOR CONTROLLER WITH SELF- BALANCING POTENTIOMETER Henry M. Schmitt, Glenside, Pa., assignor to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn, a corporation of Delaware Application March 29, 1949, Serial No. 84,206

(Cl. 121l1) 3 Claims.

This invention relates to controllers of the type which include a potentiometer as a part thereof. In such controllers, a, measuring instrument senses variations in a variable in response to which variable the control is to be exercised. The measuring instrument converts these Variations to a variable voltage which is compared to the stable voltage of a primary cell. Any diiTerence between the voltage of the measuring instrument and the voltage of the primary cell is applied to a slide wire and may be utilized to cause movement of a motor to adjust the slide Wire and rebalance the potentiometer circuit. Such controllers are very accurate but their accuracy is limited due to variations, which may occur over a period of time, in the voltage of the primary cell. The factors causing this variation are unpredictable but include the ambient temperature. To insure that the voltage of the primary cell is at its calibrated value, a resistance in series with the primary cell is adjusted periodical- 1y to bring the potential drop across the slide Wire of the potentiometer to some standard value in order to maintain the accuracy of the in- -strument. This adjustment of resistance is known as standardizing and is performed either manually or automatically. A standardizing switch is provided which disconnects the potentiometer circuit from the primary cell, which is normally connected to the potentiometer network, and connects the potentiometer circuit to a standard cell, which aifords a standard source of voltage for comparison during standardizing.

Such controllers, particularly circular chart, electronically operated controllers work on a narrow throttling range. In the case of an oil-fired, high temperature installation, during standardizing, particularly during manual or push button standardizing, the final control element, 1. e. the valve controlling the supply of fuel to the :furnace, may close and then open rapidly. In such event the fuel oil squirts into the hot furnace and may cause a big, explosive puff. If the furnace fiame is extinguished by this puff or otherwise, in cases in which there is provided a safety device responsive to the existence of a flame and operative upon the extinguishment of the flame, the safety device may cause the final control valve to close. Such closure of the final control valve causes attendant difliculties due to the ne 'cessity of reopening it.

A primary object of this invention is to provide an interlock between the standardizing switch and the supply of power to the motor driving the final control element so that the final control element is locked throughout the standardizing operation, in the position which the final control element had assumed prior to the initiation of standardizing.

A more specific object of this invention is to provide such an interlock comprising an electric switch connected to or ganged with the manually or automatically operated standardizing switch, a solenoid or like electric motor controlled from a source of electric power connected in circuit with said switch, and a valve in the air line which leads from the air-operated controller to the air-operated motor which moves a final control valve or like element.

One type of such controller comprising: a measuring element, such as a thermocouple; a self-balancing potentiometer for sensing any change of voltage generated by the thermocouple in response to the heat to which it is exposed; an electric circuit wherein the unbalanced D. C. current of the potentiometer is transformed into a pulsating current of one phase or of opposite phase depending upon the sense of unbalance of the potentiometer circuit; an amplifier which strengthen the pulsating current; a reversible electric motor operated by the amplified, pulsating current in one direction or the other so that the motor rebalances the potentiometer circuit; an indicating, recording and or controlling mechanism operated by the motor; an air-operated controller having a flapper valve also operated by the rebalancing motor which controller, in turn, controls the operation of an air-operated motor connected to a final control element, for example, a valve controllin the supply of fuel to a furnace in which the measuring instrument is located. The voltage generated by the thermocouple or other measuring instrument is compared to the voltage of a primary cell by applying both voltages to a slide wire.

Of the drawings:

Fig. 1 is a schematic illustration of the measuring and control apparatus.

Fig. 2 is a side elevation or outside view of a portion of the controller showing the manuallyoperated standardizing switch.

Fig. 3 is an electric circuit diagram of a modified form of controller having automatically, periodically-operated standardizing.

Fig. 4 is a side elevation with parts in vertical cross section of the standardizing switch and the periodically-operated motor therefor.

Fig. 1 schematically illustrates a self-balancing potentiometric controller for measuring, indicating, recording and controlling the value of a condition, for example, the temperature in a furnace it. Furnace Hi may be heated by a burner H controlled by a valve I2 driven by an air-operated motor it.

A thermocouple hi responsive to the temperature within the furnace It operates in conjunction with a potentiometer network, generally; designated 15, and a slide wire assembly, generally designated It, to form a self-balancing potentiometer device. Upon a change in temperature within the furnace I5, an unbalanced direct current (D. C.) is produced in one direction or in the opposite direction depending upon the sense of unbalance of the potentiometer system. This D. C. is applied to an amplifier unit, generally indicated I1, having a vibrator or equivalent device, a transformer, and amplifying electronic tubes.

The D. C. supplied to the amplifier i1 is converted by the vibrator into pulsating current of one phase or of opposite phase depending upon the sense of unbalance of the potentiometer system. This pulsating current is detected and amplified by the transformer and is further amplified by the amplifying electronic tubes. The output from the amplifying tubes is supplied to motor-driving electronic tubes for operating, in one direction or the other, a reversible electric motor, generally designated 24. Reversible motor 26 operates a driving mechanism, generally designated 25. The drive mechanism operates the slide wire assembly 16 to rebalance the potentiometer system and also operates indicating and recording means for indicating and recording the temperature existing within the furnace IS. The drive mechanism also operates a controller, generally designated 25, which take the form of an air-operated controller such as is illustrated in U. S. Patent 2,125,081, granted July 26, 1938, to C. B. Moore. This con troller 26 operates to control the valve I 2 to maintain the temperature within the furnace it at the desired normal value.

The potentiometer network 15 may comprise three resistances 30, 3! and 32 connected in series. These resistances are preferably formed of manganin wire having substantially no temperature resistance coefiicient and are used for calibration. Connected in parallel with resistances 30, ill and 32 is a primary cell 33, which may be a dry cell, and a dual vernier rheostat comprising movable resistances 34 and 3.5 and electrically-connected, stationary, sliding contacts 3t and 31, associated respectively with the resistances 34 and 35. The dual vernier rheostat may be manually operated by a knob 285. A pin 38 has a direct mechanical connection with the contact and a lost motion connection with the contact 31. Upon initial movement of the knob 285, the resistance 34 is first moved and then the resistance 35 is moved thereby providing vernier adjustment. This is explained in greater detail in connection with Fig. 2.

Cell 33 and the rheostat are connected in series with each other. Resistances 3 and 35 may be of copper or other material having a positive temperature resistance coefficient for compensating for temperature changes in the cell 33. Also connected in parallel with resistances 3G, 3! and 32 are two resistances 39 and 4!! which, in turn, are connected in series with each other. Resistance 35 is preferably made of copper or other material having a positive temperature resistance coefficient. Resistance 40 may be formed of manganin having substantially no temperature resistance coefficient. Resistance 59 compensates for temperature changes at the cold junction of the thermocouple [4, it being so connected into the network that the voltage drop across it is added to the thermocouple E. M. F. Resistance 2-9 is utilized primarily for standardizing purposes and has a resistance value such that the voltage drop across it is equal to the voltage produced by the standard cell 245.

Slide-wire assembly is may include a cable drum t3 suitably rotated by the drive mechanism 25 through acable 44. The term cable is used in a generic sense to cover similar arrangements such as wire, violin string, rope, cord, etc. The cable drum 53 is provided with an adjustment for adjusting the diameter or circumference thereof. The cable drum 43 operates through an adjustable connection to move a contactor 5. The contactor 45 engages a slide-wire in the form of a resistance wire it which is space wound around an insulated core 41. Preferably the slide-wire 46 and the core 41 are formed of the same material such as manganin wire having substantially no temperature resistance coefficient. The contactor 45 also engages a collector bar in the form of a resistance wire 28 space wound around a core 49 and completely electrically conductive with respect thereto throughout their lengths. Preferably, the resistance wire 53 and the core 49 are formed of the same material, such as manganin wire, as the resistance wire 4% and core 41. The slidewire assembly is provided with a terminal block 58 having terminals 5|, 52 and 53. The collector bar 48 is connected to the terminal 5!. One end of the resistance wire 33 is connected to the terminal 52 and one end of the core is connected to the terminal 53, the other ends of the resistance wire G6 and the core 41 being connected together. Connected across the slidewire it and core 41, that is in shunt with the slide-wire 56, is a resistance 5 also formed of manganin wire. The terminals 52 and 53 are connected to the ends of the resistance elements 3| of the potentiometer network l5 and thereby the slide-wire resistance A6, the resistance 56 and the resistance 3! are all connected in parallel. A two position switch generally designate at 51 is utilized for performing two functions, namely, for connecting the potentiometer circuit network for normal operation (run) and for standardizing the potentiometer system (standardize). schematically, this switch 21 may coinprise switch arms 58, 59, 69 and 5: and 69 electrically insulated from each other and all operated by a common operating member The switch arms 58 and 59 carry contacts 53 and respectively. The switch arm as carries contacts G5 and 55 and the switch arm 6: carries contact 61. Contact 63 engages a contact when the switch 51 is moved to the standardize position. When switch 51 is in the run position the contact 53 does not engage contact When the switch 5i is in run position the contact t5. engages both contacts is and 1!. When the switch is moved to the standardize position contact E i disengages contact 1!. Contact engages contact 14 when the switch 5'5 is in run position. When the switch 51 is moved to standardize position, contact 65 disengages contact and contacts 55 and 68 engage contacts ?2 and 13, respectively. When the switch 51 is in the run position contact 51 does not engage contact 18. Contact 61 engages contact 16 only when the switch 51 is moved to standardize position.

For the purposes of illustration it is assumed that the thermocouple I4 is an iron-constantan thermocouple. The lead I extending from the iron element of the thermocouple to the positive cold junction terminal IOI is an iron lead. The lead I92 leading from the constantan element of the thermocouple to the negative cold junction terminal I03 is made of constantan. The hot junction of the thermocouple is, therefore, in the furnace I0 and the cold junction is at the cold junction terminals IM and I03. The positive cold junction terminal MI is connected to the contacts I5 and 61 of the switch 5?. The contact M which normally engages the contact 65 is connected to the terminal 5I and hence to the collector bar of the slide-wire assembly i6. Accordingly, the positive lead of the thermocouple is connected through the transformer and the vibrator to the collector bar 48 of the slidewire assembly I6 when the switch 5'! is in the run position.

The cell 33 operates to produce an E. M. F. between the contactor of the slide-wire assembly I5 and the point I09 in the potentiometer network I5 which opposes the E. M. F. produced by the thermocouple I4 and the value of the cell E. M. F. produced between these points is regulated by the position of the contactor 45 along the slide-wire resistance 45. As the contactor 45 of the slide-wire assembly is moved in a clockwise direction the cell E. M. F. between the contactor 45 and the point I09 is increased and as the contactor 45 is moved in a counter-clockwise direction the cell E. M. F. is decreased. When the potentiometer system is balanced the E. M. F. produced by the cell between the contactor 45 and the point I09 is equal and opposite to the E. M. F. produced by the thermocouple I4 and, therefore, no current flows in the thermocouple circuit.

Upon an increase in temperature within the furnace In the E. M. F. produced by thermocouple I4 becomes greater than the E. M. F. produced by the cell 33 between the contactor 45 and the point I09 which causes a D. C. current flow from the positive cold junction terminal lliI through wire terminal I04 to the terminal N2 of the transformer It to the terminal 81. From the terminal 81 the D. C. current flows through contacts and 14 of the switch 51, terminal 5| of the slide-wire assembly, collector bar 48, contactor 45, slide-wire resistance 45, potentiometer network I5 to I00, and then through contacts 'II, 64 and 10 of the switch 57, resistance I08, checking terminal I01, jumper we, and checking terminal I95 to the negative cold junction terminal 53. This produces a D. C. current in one direction which is detected and amplified by the amplifier IT to operate the reversible motor 24 which through the drive mechanism 25, cable 44 and cable drum 43 moves the contact 45 in a clockwise direction to increase the cell E. M. F. between the contactor i5 and the point 109 to oppose equally the M. F. developed in the thermocouple. When this occurs the flow of D. C. current is stopped and the potentiometer system becomes balanced.

Upon a decrease in temperature within the furnace It the E. M. F. produced the thermocouple it becomes less than E. M. F. produced between the contactor 45 and the point I69 by the cell 33. This causes a D. C. current to flow in the opposite direction from the contactor 45 through collector bar 48, terminal 5| of the slide-wire assembly I5, contacts I4 and 65 of the switch 51, to terminal 81 of the vibrator IT. This D. C. current then flows from the terminal I12 through positive checking terminal I04, positive cold junction terminal I0 I, thermocouple I4, negative cold junction terminal I03, negative checking terminal, jumper, negative checking terminal, resistance I08, contacts I0, 64 and 'iI of the switch 51 and point I09 through the potentiometer network I5 to the contactor 45 of the slide-wire assembly I6. This flow of D. C. current is detected and amplified by the amplifier IT to operate the reversible motor 24 in the opposite direction which in turn operates through the drive mechanism 25, cable 44 and cable drum 43 to move the contactor 45 of the slide-wire assembly I6 in a counter-clockwise direction. This decreases the E. M. F. produced by the cell 33 between the contactor 45 and the point I09 to a value which equally opposes the E. M. F. produced by the thermocouple I4 whereupon the potentiometer system again becomes balanced and no D. C. current flows.

Accordingly, upon a change in temperature within the furnace I0 an unbalanced D. C. current is caused to flow in one direction or the other depending upon whether the furnace temperature increases or decreases. This unbalanced D. C. current flow is detected and amplified by the amplifier I! to operate the reversible motor 24 which in turn operates through the drive mechanism 25 and the slide-wire assembly I6 to reduce the unbalanced D. C. current flow to zero. The positions of the contactor 45 of the slidewire assembly I5 and of the indicating, recording and controlling parts mechanically coupled thereto represent the actual temperature value xisting within the furnace I0.

The motor 24 is, therefore, rotated in one direction or the other as the potentiometer circuit is unbalanced in one direction or the other and the speed of operation of the motor in either direction is dependent directly upon the amount of potentiometer unbalance. As the potentiometer circuit is restored to balance following an unbalance thereof, the braking action on the motor is increased so that rotation of the motor is rapidly stopped when the potentiometer is rebalanced without any hunting.

When the motor is operated at maximum speed the inductance of the motor control winding is increased, for at this time the D. C. current flow through the motor control winding is at a minimum. Also, at this time the alternating current flowing through the motor control winding is completely a 60 cycle alternating current instead of the cycle current flowing through the motor control winding when the potentiometer is balanced and the motor is stationary. Due to this decrease in frequency and due to the increase in inductance in the motor control winding when the motor is operating at a maximum speed, the condenser 2II connected in parallel with the motorcontrol winding still provides a parallel resonant circuit so that the impedance offered by this parallel resonant circuit is substantially resistive to maintain the plate voltage in phase with the plate current; As pointed out above a resonant condition occurs when the motor is stationary with th potentiometer balanced so that under that condition the plate voltage and plate current are also in phase. Proceeding from the condition wherein the motor is station ary to a condition wherein the motor is operating at maximum speed the circuit formed by the condenser ZII being connected in parallel with the motor control winding remains substantially resonant so that the plate current and the plate voltage are substantially always in phase.

As the rotor 20! of the motor 24 rotates the flux produced by the-power windings 202 and 283 is distorted by the rotor rotation to cause some of the flux produced by the power windings to link the control windings 204 and 2 .25. This induces additional voltage in the control windings 2M and 265 which is of the same phase and frequency as the voltage normally produced therein to assist the voltage in the control windings 2M and 2%. This induces current in the control windings through the low resistance local path of the parallel resonant circuit thereby giving a relatively large current flow even though only a few turns of the control windings are linked by the distorted fiux. The greater portion of the current and hence the power for the control windings when the motor is operating at maximum 7 speed is induced by this transformer action so that the tubes may only conduct a relatively small portion of the total current or power required to energize the control windings. The amount of current induced by transformer action is proportional to the speed of rotation of the rotor. This action tends to increase the life of tubes.

The motor rotor operates a pinion Zt-i w ich drives a gear H5. The gear ZIE carries a gear H6 and a cable drum 217. The cable is which drives the contactor 45 of the slide-wire assembly I6 is fastened to the cable drum 2 l i and passes over a tension pulley 2|8 carried by a lever 219 and urged downwardly by a spring 225. The cable M also passes over pulleys 22! and 222 and is fastened to cable drum G3. The spring 225 urging the pulley 2I8 downwardly maintains a predetermined tension in the cable 4 3 so that the relative positions of the cable drum 2i? and the cable drum 43 will always remain fixed. The lever 219 is provided with an enlar ed hole 2.2? through which extends a stationary pin 223, the pin cooperating with the hole to act as a stop for limiting the movement of the lever 22%. The gear 2 i B operates a gear sector 223 which in turn operates a pen arm 22% for recording the temperature values on a chart 225 and in the case of a control instrument also operates an arm 225 for controlling the control device 25. 225 is rotated at a constant speed by the chart drive motor [20.

The arm 22B operates a flapper mechanism 236 carried by the body portion 23! of the control device 26. The control device 25 is equipped with dials 232 and 233 for adjusting the throttling range and the rate of reset of the control device. Air under pressure is supplied through a pipe 234 to a pneumatic relay mechanism 235 and a pipe 236 controlled by the control device 2G regulates a pilot valve contained in the relay mechanism to produce a pressure in a pipe 23! in accordance with the temperature condition existing within the furnace H). The pressure in the pipe 231 is conducted through a pipe 238 to a pneumatic motor I 3 which operates the valve [2 and the pressure is also conducted through a pipe 2:10 to the control device 26 in order to provide the follow-up and reset action in the control device 26.

Upon an increase in temperature within the furnace ID the motor 24 is operated in the counter-clockwise direction to drive the gear 2|5 in a clockwise direction. Rotation of the gear 215 in the clockwise direction causes the con The chart tactor of the slide-wire assembly 16 to rotate in a clockwise direction to rebalance the potentiometer system. Clockwise rotation of the gear 215 also causes the gear sector 223 and the pen 224 to rotate in a counter-clockwise direction to record on the chart 228 the increase in temperature which has taken place within the furnace l8. Counter-clockwise rotation of the gear sector 223 operates through the arm 226 and flapper assembly 230 to actuate the control device 26 to position the relay mechanism 235 to decrease the pressure transmitted to the pneumatic motor it of the valve l2. This moves the valve I2 toward the closed position to decrease the supply of fuel to the furnace to for the purpose of reducing the temperature of the furnace to the desired normal value. The decrease in pressure transmitted to the diaphragm motor I3 is also transmitted to the control device 25 to provide a follow-up action for causing the valve [2 to be positioned in accordance with the temperature within the furnace. This decrease in pressure transmitted to the control de vice 28 also operates through the reset mechanism thereof to additionally position the valve i2 towards a closed position if the temperature change is caused by a change in temperature load in the furnace extending over a substantial period of time. Upon a decrease in temperature within the furnace ii] the opposite action takes place, the contactor d5 of the slide-wire assembly It being moved hr a counter-clockwise direction, the pen arm 22% being moved in a clockwise direction to record on the chart .225 the decrease in temperature and the control device 26 being actuated. to cause proportionateopering of the valve l2 to increase the supply of to the furnace ii) for restoring the furnace ten perature to the desired .value.

The gear 215 is formed with an abutment 252 which engages the drive pinion when the gear 2H5 is rotated to either extreme p0 t on. In engaging the drive pinion 2H: the abu nent 242 stalls the motor 25; and prevents overtravel of the contactor as of the slide sembly [6 0nd over-travel of the pen a- 2 Due to the relatively high inductive reactance to resistance ratio of the motor due to the lack of transformer action when the motor is stalled, is found that when the motor is thus stalled the current flow through t e motor: is less than when it is actually running so that the motor 24 does not heat up under these stalled conditions. By reason of this arrangement use of limit switches or equivalent devices for stopping operation of the motor 2:? is entirely eliminated.

In order to standardize the potentiometer circuit, that is, adjust the dual Vernier battery rhecstat to maintain a predetermined potential across the slide-Wire assembly, the three-way switch fil is moved downwardly to the stand rdiZe position. Engagement of contact with contact 16 connects the negative end of the standard cell 245 through wire its to the terminal [T2 of the transformer. Movement of the con tact out of engagement with the contact it breaks the connection from the slide-wire assembly to the terminal 6? of the vibrator and engagement of contact 65 with contact i2 connects the negative end of the standard cell through resistance 24! to the terminal ii? of the vibrator i8. Movement of the contact as into engagement with the contact 73 connects the negative end of the standard cell 245 through resistance Ml to the point I99 of the potentiometer circuit. Movement of the contact 64 out of engagement with the contact H breaks the connection between the negative thermocouple lead and the point [B9 of the potentiometer circuit, In other words, when the three-position switch 51 is moved to the standardize position, the collector bar 48 of the slide-wire assembly [6 and the thermocouple it are disconnected from the circuit and the ampliher I? including the vibrator and the transformer is connected across the resistance 2 51 which in turn is connected in series with the negative end of the standard cell 245 and the point is9 of the potentiometer circuit.

The voltage produced by the standard cell op poses the voltage drop across resistance 49 produced by the cell 33 and any difference in the voltages causes a current flow through the resistance 241 in one direction or the other depending upon which voltage is greater. This current flow through the resistance 241 is detected and amplified by the amplifier H to cause the motor 2d to rotate in one direction or the other. This rotation of the motor '24 indicates that the current produced by the cell 33 is different than that produced by the standard cell 245. The dual vernier rheostat in series with the cell 33 is Vernier rheostat in series with the cell 33 is then adjusted until rotation of the motor 2 3 ceases. When this occurs the current produced by the cell 33 produces a voltage drop across the resistance 48 which is equal to the voltage produced by the standard cell 268 and the potentiometer circuit has been standardized. Following this standardization the switch El is then returned to the run position for normal operation.

The manual adjustment of the dual Vernier rheostat is accomplished in a manner best shown in Fig. 2. The standardizing resistance, which is shown diagrammatically in Fig. 1, is shown in detail in Fig. 2 as consisting of a Vernier rheostat which is made up of a fine resistance 35 and a coarse resistance 34 which resistances are mounted on a shaft 299 that extends to the left from the wall of the mechanism-supporting casting 72A. The arrangement is, that the fine resistance is adjusted directly and the coarse resistance is rotated by the fine resistance by means of a pin 38 which is attached to the fine resistance 35 and extends to the right therefrom through a slot that is formed in the disc supporting the coarse resistance 34. The resistances are engaged by a pair of electrically connected contacts 36 and 3'! which serve the purpose of connecting these resistances in the potentiometer circuit and also, by their frictional engagement with the resistances serve to prevent accidental rotation of the resistances.

It was pointed out above in the description of the wiring diagram that the standardizing switch 57 could be moved to bring contacts 6] and it and contacts 53 and 68 into engagement with each other in order to standardize the instrument. The manner in which this is accomplished is shown in Figure 2.

Mounted on the instrument casing 72A, adjacent and above the switch 51, is a leaf spring 278 that is engaged and moved by an adjustable screw 279 which, in turn, is mounted on a lever 289. In order to move the switch to its standardizing position, the lever 280, which is pivoted at 281 is moved in a counter-clockwise direction to bring the leaf 218 into engagement with the button or common operating member 62 by engagement between a pin 282 that projects from the lever 289 and a cam surface 283 that is formed on a shaft 284. This shaft is journaled for axial and rotative movement in the frame 12A in suitable bearings. Shaft 284 may be rotated and moved axially by means of knob 285 on its right hand end, in standardizing the instrument, the knob is moved to the left in Fig. 2 to move the operating button or knob 62 of the switch 5? downwardly. Continued leftward movement of the knob 285 will bring a clutch plate 281 on the left end of the shaft 284 into engagement with a second clutch plate 289 on the shaft 286 of the standardizing rheostat 34-35. After the clutch plates have been connected and the switch 5'! has been thrown downwardly, rotation of knob 285 will adjust the resistances 34 and 35 to standardize the instrument.

If the voltage of the primary cell 33 has seriously decreased before the standardizing operation is understaken, connecting the rebalancing motor 24 across the standard cell 245 may cause the final control valve i2 to move to closed position, as it should, but may also permit the final control valve [2 to thereafter reopen to a greater or less extent. Such a reopening of valve 12 would permit a flow of fluid through the nozzle H to the hot furnace I 0 Where the fuel would be ignited with more or less explosive violence. In the case in which the furnace i9 is protected by a light sensitive device, such an explosion might cause the furnace IE] to set down with attendant difficulties in restarting. Such explosions are prevented by the interlock of this invention. This interlock consists of the individual switch contacts 11 and 19 which are mounted adjacent the switch 57 so as to be bridged by the bridging contact 78 mounted on the switch arm 68. Contact I9 is connected directly to the main alternating current lead H8. Switch contact ll, however, is connected by wire 88 to one side of a solenoid 8| or other electric motor. Motor 8i has operating connections with a valve 82 in the conduit 238 which supplies air from the controller 235 to the air-operated motor l3 which drives the final control valve l2. The other side of solenoid 81 is connected by wire 33 with the other alternating current main H9. Thus, it will be seen that when switch 57 is operated to permit standardizing of the instrument in the manner described, solenoid 8! is energized and operates to close valve 82 and thereby lock the air in motor [3. This causes valve I2 to remain in the closed position which it has assumed at that time. Therefore, any danger of admission of fuel to furnace l9 and the consequent hazard of explosion is avoided.

Figs. 3 and 4 show a modification in which the standardizing operation is performed automatically.

As has been pointed out in connection with Figs. 1 and 2, because of the fact that the output voltage of the cell 33 changes with use and changes with unpredictable factors, such as the ambient temperature to which it is subjected, it is necessary to adjust the variable resistance 3 from time to time in order to maintain the tie sired calibration of the potentiometer instrument. At such times as the instrument is to be recalibrated or standardized, the movable con tact 5 of the switch 51A is shifted out of engagement with the stationary contact I and into engagement with contacts 2, 4, 1'! and 19 thus effecting disconnection of the thermocouple it from the circuit and connection of the standard cell 24am opposition to the potential drop across the resistance 5. Simultaneously, the movable member of switch 51A engages the stationary contact 2 to connect a resistance 2 37 in shunt to the input terminals of the electronic device I?. The resistance 24'! is utilized for the purpose of desensitizing the electronic device I 7 because the opposed potential drops connected to the input circuit of device I! when the instrument is in its standardizing position are of much greater magnitude than are the opposed potential drops which are impressed upon the input circuit of the device I! when. the instrument is in its normal measuring condition. Consequently, the extreme sensitivity of the electronic device required in the normal measuring condition of the potentiometer is not required nor desired in the recalibrating or standardizing position of the instrument.

Simultaneous with the adjustment of switch member 5 into engagement with contacts 2, i, 7'! and 79, the output shaft of the reversible motor 2% is connected to the variable resistance 3 for effecting adjustment of the, latter in accordance with the angular position of said shaft. To this end, a clutch mechanism indicated generally by the reference character 399 is provided between the output shaft of motor 2% and the resistance 3. The clutch mechanism 368 may be of the type disclosed in Patent 2,377,474 issued to A. F. Allwein on June 5, 1945. Such clutch mechanism is illustrated in detail in Fig. 4.

In the normal measuring condition of the potentiometer instrument, the output shaft of the reversible motor 2 3 is connected to the contact 45 for adjusting the latter along the length of the slidewire resistance ed. The rotation of the reversible motor 2t accomplishing this result does not during this condition of operation produce any adjustment of the variable resistance 3. The clutch mechanism 368 includes time operated mechanism, however, which periodically shifts the movable member 5 of the switch 57A out of engagement with contact I and into engagement with the contacts 2, t, 77 and i9 and simultaneously connects the output shaft of the motor to the variable resistance 3. This is the standardizing condition'of the potentiometric instrument. If the potential drop across the resistance 46 is then not exactly equal to the potential of the standard cell 2 56, indicating that the potentiometric network needs recalibration, the electronic device ll detects such inequality of the opposed potentials and causes selective energization of the reversible motor 23 for rotation to effect adjustment of the variable resistance 3 in the proper direction to restore the correct calibration condition of the potentiometric instrument. The motor 2d also operates to adjust the contact 45 along the slidewire resistance 2-8 during this time, but such adjustment in no way aifects the standardizing operation inasmuch as the thermocouple circuit is then opened at the switch 51A.

In Fig. 4, is shown the details of the clutch mechanism 3653 of Fig. 3. In Fig. 4 the standardizing resistance 3 is shown as consisting of a vernier rheostat which is made of a fine resistance 34 and a coarse resistance 35, which resistances are rotatably mounted on a shaft 3M that extends from the mounting wall 72A which supports the mechanism. The parts are so arranged that the fine resistance is adjusted directly and the coarse resistance is rotated by the fine re.- sistance by means of a pin 38 which is attached to the support for the fine resistance 34 and extends to the right therefrom through a slot which is formed in the resistance 35. The resistances are engaged by a split contact 35" 37 which serves the purpose of connecting these resistances in the potentiometric circuit and due to its frictional engagement with the resistance disks also serves to prevent accidental rotation of the latter. 7 v

The resistances are adjusted relatively to the contact 3631 by means of clutch 309 which comprises a driven clutch disk 289 that is formed as a part of the resistance 34 and a driving clutch disk 287 which is also rotatably mounted on the shaft 3M, and is attached to a gear 392. The gear 302 meshes with a pinion 3ll3driven by the motor 24 so that, as the latter rotates, the gear and the driving disk 281 also rotate and at predetermined times when the clutch disks 231 and 239 are in engagement with each other, serve to adjust the resistance 3. Driving disk 287 is moved into or out of engagement with driven disk 2% by means of a lever 33-; that isattached to a shaft 395. The upper end of this leverhas a roller 3% which engages the left face of the clutch disk 28? to move this disk to the right into engagement with the disk 289, or engages the right face of gear 392 to separate the disks. During the movement of the disk 28?, gear 302 will slide along the pinion 363, the latter being wide enough to permit this to take place. The shaft 595 also has attached to it a second lever 30?; the two levers and shaft being biased in a clockwise, or clutch closing direction by means of a spring 308. The lever 507 also has an 'ex'- tension 399 projected rearwardly from it which serves, by means of a pin 318, to throw the standardizing switch 57A from the position shown in Figs. 3 and 4 to a position to bring contact 5 into engagement with contacts 2, A, 77 and .79 whenever the clutch is closed. The switch 51A is shown in Fig. 4 as being of a leaf type, am is mounted on a support 3| 1 which is adapted to be fastened as shown at 352 to the wall 72A of the mounting.

The movable element, generally indicated at 5, l

of the switchS'l has attached to it a bridging contact 78 which engages a pair of relativelystationary contacts 7'1 and 19 mounted on leaf spring contacts.

Normally, during the operation of the instrument, the standardizing mechanism parts are in a position shown in Figs. 3 and 4, and are held in that position by means of engagement be tween a roller 3|3 and a cut out portion 3 M on the upper end of the lever 39?. The roller 3tl3 is mounted on a crank 3!?) that is attached to a shaft 3H5. This shaftBlfiis periodically rotated, by any one of a number of suitable means. Such means, suitable for causing the shaft SIG to make and complete a rotation at regularly recurring intervals are well known to those skilled in the art, and need not be described in detail here since they form no part of the present invention. It is, therefore, only necessary to state here thatthe shaft 316 is periodically given a complete rotation, during which rotation the standardizing switchfilA is shifted to itsstandardizingposition and the clutch plates 28'! and 28s are held together so that the motor 24 is made operative to adjust the resistance 3 if such adjustment is necessary to obtain a balance of the potentiometric system, indicating proper'in strument calibration.

During the standardizing operation, motor 24 moves in response to any difierence between the voltage of primary cell 33 and the standardizing cell 246 to rebalance resistor 3. In order to prevent the valve 12 from moving in response to this movement of rebalancing motor 2%, an interlock is provided between the standardizing switch 51A and the valve l2 controlling the supply of fuel to the furnace nozzle II. This interlock consists of the bridging contact 18 on the movable member 5 of switch 51A. Contact 18 engages contacts I1 and 19 in the standardizing position of switch 51A. Engagement of switch contac s 11 and 19 with bridging contact 18 closes a circuit from the main power lines H8 and H9 through lead wires 80 and B3. Solenoid 8| is ener i ed and closes valve 82 in the air supply line to a tube 238 which supplies air to the motor l3 wh ch onerates the valve 12. Since the air is thus prevented from escaping from the motor l3. the valve I! must remain in closed position until the airsup lv line 238 is again reopened.

While in accordance w th the rovisions of the statutes, I have illustrated and described the best form of the invention now known to me. it will be apparent to those ski led in the art that changes may be made in the form of the apparatus disclosed without departing from the spirit of the invention as set forth in the appended claims, and that in some cases certain features of the invention may sometimes be used to advantage without a corresponding use of other features.

Having now described my invention what I claim as new and desire to secure by Letters Patent is as follows:

1. In a controller having a measuring instrument responsive to a variable to be controlled, 2. self-balancing potentiometer normally connected to said measuring instrument and having a rebalancing motor and a balancing resistance and a standardizing switch, an air-controlling pilot valve governed by the movements of sa d motor in res onse to the variations sensed bv said measurine instrument and an air-operated motor controlled by the pilot valve, the combination including, periodically operating means for moving said standardizing switch to disconnect the measuring instrument and to connect a standard cell to said potentiometer, a clutch operated by said periodically operating means to connect said rebalancing resistance and said standard cell to said rebalancing motor when said standardizing switch is moved to standardizing position, a power-conducting circuit adapted for connection to a source of power, a circuit-making-and-breaking-device controlling the flow of power through said power-conducting circuit, said device being moved to render said circuit power-conducting when the standardizing switch is moved to standardizing position, a power-operated motor connected in said circuit and normally having a first position and operative to a second position when said circuit is conducting power, and a valve driven by said motor to closed position when said motor is in the second position, said valve being located in the air-conducting connection to the air-operated motor so as to prevent the entrance or exit of air to or from said air-operated motor when said valve is closed.

2. An interlock between the standardizing switch of a self-balancing electric potentiometer and an air-operated motor for operating a final control element under the control of said potentiometer, which interlock includes, a valve controlling the inlet and exhaust of air to said airoperated motor for operating a final control element, an electric motor having operative engagement with said valve, an electric circuit including said electric motor and adapted for connection to a source of electricity, relatively stationary switch contacts, a movable switch contact movable with the standardizing switch of the potentiometer to connect said stationary contacts into sa d electric circuit when the standardizing switch is moved to standardizing position, a rebalancing motor forming part of said potentiometer, a standardizing res stance forming part of said potentiometer, a normally disenga ed clutch operatively located between said rebalancing motor and said standardizing resistance, and means for periodically engagin said clutch and causing said movable and stationary switch contacts to engage to thereby cause said rebalancing mo or to move said standardizin res stance to balanced position.

3. In a controller having a measuring instrument responsive to a variable to be controlled, a self-balancing potentiometer normally connected to said measuring instrument and having a standardizing resistance and a standardizing switch, an air-controlling pilot valve governed by the movements of said potentiometer in response to the variations sensed by said measuring instrument and an air-operated motor controlled by the pilot valve, the combination including, operating means for moving said standardizing switch to disconnect the measuring instrument and to connect a standard cell to said potentiometer, a clutch operated by said operating means to connect said standardizing resistance to said operating means when said standardizing switch is moved to standardizing position, a power-conducting circuit adapted for connection to a source of power, a circuit-making-and-break ng device controlling the flow of power through said powerconducting circuit, said device being moved to render said circuit power-conducting when the standardizing switch is moved to a standardizing position, a power-operated motor connected in said circuit and normally having a first position and operative to a second position when said circuit is conducting power, and a valve driven by said motor to closed position when said motor is in the second position, said valve being located in the air-conducting connection to the air-operated motor so as to prevent the entrance or exit of air to or from said air-operated motor when said valve is closed.

HENRY M. SCHMI'I'I.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,080,789 Ryder May 18, 1937 2,301,898 Luhrs Nov. 10, 1942 2,390,902 Vollrath Dec. 11, 1945 2,423,479 Caldwell July 8, 1947 

